1. Field of the Invention
The present invention relates to light valves, and in particular, to pixel cell arrays for silicon light valves which include a reflective metal layer positioned underneath inter-pixel regions.
2. Description of the Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These display devices typically include a light source which passes light through a light valve.
One of the methods for producing colors in a liquid crystal display is to sequentially project light having a wavelength corresponding to a primary color onto a single light valve. Color sequential light valves create a spectrum of color within the range of the human perception by switching between a set of discrete primary colors. Typically, red, green, and blue are the primary tri-stimulus colors used to create the remaining colors of the spectrum.
Specifically, during projection of each primary color, the light intensity is modulated such that combination of the intensities of the primary colors in sequence produces the desired color. The frequency of switching between the primary wavelengths by the light valve should be sufficiently rapid to render discrete primary states indistinguishable to the human eye.
Two factors dictate the minimum frequency necessary for switching. The first factor is the ability of the human eye to detect the discrete primary colors (e.g., red, green, blue). At slower than ideal switching speeds, the human eye will detect a flicker and the primaries may not blend.
The second factor determining the frequency of switching is the video refresh rate. During display of video images, the individual frames must be refreshed at frequencies undetectable to the human eye.
The net frequency of switching demanded by the combination of sequential color blending and video refreshing is beyond the capabilities of light valves that utilize thick ( greater than 1 xcexcm) liquid crystal (LC) transducers. However, thin ( less than 1 xcexcm) liquid crystal transducers have been successfully fabricated. These thin LC transducers demonstrate adequate color sequential blending at video refresh rates. One example of such a thin LC transducer pixel cell structure is disclosed in U.S. Pat. No. 5,706,067, to Colgan et al.
In general, the conventional thin LC transducer pixel cells possess enhanced responsiveness due to the decreased volume of liquid crystal material between the top and bottom plates. A smaller volume enables the liquid crystal to shift orientation more quickly and in response to a lower applied voltage.
FIG. 1 shows a cross-sectional view of adjacent thin LC transducer pixel cells in a conventional light valve. Light valve portion 100 comprises adjacent pixel cells 110a and 110b having liquid crystal (LC) material ill sandwiched within gap 106 between a top plate and a bottom plate. Top plate 102 is composed of a translucent material, typically glass. The bottom plate is formed by the reflective metal pixel electrodes 112a and 112b of adjacent pixels 110a and 110b, respectively.
Pixel electrodes 112a and 112b are separated and electrically isolated by trench 118. Pixel electrodes 112a and 112b lie on top of an upper intermetal dielectric layer 128 that is one component of interconnect scheme 104. Interconnect 104 overlies capacitor structures 118a and 118b formed within underlying silicon substrate 105. Underlying capacitors 118a and 118b are in electrical communication with pixel electrodes 112a and 112b, respectively, through metal-filled vias 140 and middle interconnect metallization layer 124 and lower interconnect metallization layer 122.
The conventional pixel array described above in FIG. 1 functions adequately in many applications. However, this design suffers from a number of disadvantages.
One problem is that light incident to array 100 may penetrate through trench 118 between adjacent pixel electrodes 112a and 112b. Intermetal dielectric layer 128 below trench 118 is substantially transparent to this incident light, which next encounters interconnect metallization layer 124. Metallization layer 124 likely bears an anti-reflective coating as a result of prior photolithographic steps. As a result, light incident to inter-pixel regions is absorbed rather than reflected, and is perceived by a viewer as a dark line. This dark inter-pixel region stands in stark contrast to the bright surrounding reflective pixel electrodes. Projection displays can in turn magnify the light reflected from pixel array to such an extent that non-reflective s pace between pixel s is readily observable and may distort the image.
Therefore, there is a need in the art for a pixel array and a process of forming a pixel array where inter-pixel regions exhibit reflectance comparable to reflectance in pixel regions.
Another problem is that the penetration of light into inter-pixel regions can cause distortion of the image displayed by the light valve. Specifically, incident light can travel through a variety of paths in the interconnect and finally enter into the underlying silicon. Penetration of incident light into the silicon substrate induces electrical currents that disturb charge stored in the underlying capacitors. As a result of fluctuation in charge at these capacitors, luminance of the pixel cells may change between succeeding write states, causing the image to xe2x80x9cflicker.xe2x80x9d This flickering reduces image quality, and may cause eye strain in a viewer.
Therefore, there is a need in the art for a pixel array and a process of forming a pixel array that substantially blocks the penetration of incident light through inter-pixel regions in to the underlying substrate.
The present invention provides a pixel array and process flow for forming the array that positions a reflective metal surface beneath inter-pixel regions. This underlying metal surface reflects incident light, thereby preventing absorption of light in inter-pixel regions giving rise to dark lines between bright reflective pixel electrodes.
A process flow for forming a pixel cell array in accordance with one embodiment of the present invention calls comprises the steps of forming a first dielectric layer over a lower interconnect metallization layer, and forming a second dielectric layer over the first dielectric layer. Next, a window photoresist mask is patterned over the second dielectric layer, the window photoresist mask masking a pixel region and exposing an inter-pixel region. A window is created in the second dielectric layer by etching inter-pixel regions to stop on the first dielectric layer, and the window photoresist mask is removed. A reflective metal layer is formed over the second dielectric layer and within the window, and the reflective metal layer is removed outside of the window. A third dielectric layer is then formed over the reflective metal layer and the second dielectric layer, and a via is formed by etching through the third dielectric layer, the second dielectric layer, and the first dielectric layer to stop on the interconnect metallization layer. Finally, the via is filled with electrically conducting material and a pixel electrode is formed within the pixel region.
An apparatus in accordance with one embodiment of the present invention comprises a plurality of reflective metal pixel electrodes formed over a dielectric layer and separated by inter-pixel regions, and a reflective metal surface positioned in inter-pixel regions underneath the dielectric layer.
The features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.